Hydrogen peroxide is known as a moderately effective disinfecting agent with some bacteriostatic properties. It is applied in the disinfection of sewage waters etc. The utilization of the reactivity of hydrogen peroxide with metal ions (Fenton reaction) is the most powerful use of hydrogen peroxide in disinfection. UV irradiation is another applicable way to activate hydrogen peroxide in disinfection. Both Fenton reaction and UV irradiation produce hydroxyl radicals to the reaction media. However, the disinfection power of hydrogen peroxide is not sufficient for most of the microbes.
Peracetic acid (PAA) is known as an effective disinfecting agent providing a rapid reduction of bacteria growth for most of the common bacteria. It is applied in the sterilization of the equipment in dairy industry. Moreover, PAA is applied in pulp and paper industry for the control of the microbial growth in process waters. In addition, peracetic acid is applied in the post-bleaching of kraft pulps after delignification and peroxide bleaching steps.
Peracetic acid is traditionally prepared via an equilibrium reaction between acetic acid and hydrogen peroxide resulting in an equilibrium solution:acetic acid+hydrogen peroxide< - - - >peracetic acid+water
This reaction occurs only when catalyzed by a strong mineral acid, e.g. sulfuric acid.
The equilibrium solution of peracetic acid is used as a disinfecting agent for example in process water applications, greenhouses, dairy industry etc.
Similar equilibria occur with formic acid and hydrogen peroxide, resulting in solutions containing performic acid (PFA). PFA solutions are proven to be more effective disinfecting agents than peracetic acid solutions. Due to instability and high reactivity of PFA the solutions of PFA are not stable. Thus, PFA solutions have to be prepared in situ. A stable disinfecting agent comparable with PFA should be desirable for applications in greenhouses, industrial and institutional (I&I) cleaning and control of microbial growth in process waters e.g. in pulp and paper (P&P) industry.
In WO 94/20424 performic acid is successfully applied in the control of microbial growth in the horticulture. A dilute performic acid solution fed into the nutrient solution or drain water prevents the growth of algae in the pipelines going to the plants preventing them from plugging. Extensive studies of the applicant have shown that the equilibrium mixtures of PAA are not applicable in such applications due to their lower reactivity towards microbes. Furthermore, the disinfection power of formic acid in the absence of hydrogen peroxide is proven to be negligible compared to peroxy acids.
The stability of peroxy acids is known to increase with increasing molecular weight. On the other hand, together with the reactivity, the disinfection power of peroxy acids increases with the decrease of the molecular weight. Performic acid (PFA) is known as the most powerful disinfection agent among the peroxy acids. Several studies have shown the effect of performic acid in the control of microbial growth. One of the earlier references is J. Hyg. Epidem. Microbiol. Immunol. (1968) 12, 115.
WO 94/20424 describes the preparation of performic acid solutions in situ by reacting formic acid and hydrogen peroxide in a molar ratio of from 1:10 to 10:1, preferably from 1:1 to 1:5. The PFA solution can be employed for preventing and combating harmful microorganisms. Typically PFA is used in an amount of 1-1000 ppm.
EP 231632 A discloses the industrial use of performic acid as a sanitizer. The performic acid solution is prepared in situ from an aqueous solution containing from 10 to 50% by weight hydrogen peroxide and a solution containing from 5 to 100% by weight formic acid by reacting the same in the presence of a catalyst, the weight ratio of hydrogen peroxide to formic acid being in the range of from 1:6 to 1:1.5.
For greenhouse disinfection purposes, the solution of PFA is prepared by mixing for example, 35% hydrogen peroxide with 15% formic acid solution. The resulting solution is then diluted with the nutrient solution and fed to the plants.
Due to the fact that performic acid solutions are explosive in higher concentrations, only low concentrations of performic acid solutions can be handled safely.
Mixtures of dilute solutions of performic acid and peracetic acid are known from the recent patent literature.
U.S. Pat. No. 6,211,237 B1 discloses a dilute disinfecting agent comprising small amounts of performic acid and peracetic acid, the total amount of these peracids typically being less than 4% by weight. The main component of the agent is hydrogen peroxide, the amount thereof typically being about 50% by weight. The agent may be used for example for disinfecting swimming pool waters. The dilute solutions of PFA and PAA with hydrogen peroxide are prepared in situ. It is also known for the man skilled in the art that peroxy acid solutions, even the equilibrium solutions of performic acid, are relatively stable at low concentrations, up to 2% by weight, for several days.
US 2004/0035537 describes a method for bleaching pulp with a solution containing peracetic acid and performic acid. Also in this application, dilute peroxy acid solutions are prepared in situ by bringing acetic acid and formic acid into contact with hydrogen peroxide at a concentration greater than 50% by weight. The ratio of acetic acid+peracetic acid to formic acid+performic acid is preferably 9 to 1 by volume. The amount of peracids in the obtained solution is very low, typically less than 2% by weight.
U.S. Pat. No. 6,284,793 B1 discloses a biocidal agent for treating ballast sea water. The biocidal agent is in the form of a solution containing peracetic acid, performic acid, acetic acid, formic acid, hydrogen peroxide and water and optionally a mineral acid catalyst and active oxygen stabilizers. Such a solution may be obtained by adding formic acid to an equilibrium peracetic acid solution, typically containing 1 to 15% by weight peracetic acid. According to this document performic acid is more effective compared with peracetic acid, but also more susceptible to decomposition and therefore formic acid is added to the solution containing peracetic acid only just prior to use. When the equilibrium peracetic acid and formic acid are used in combination, the formic acid being added directly to the peracetic acid or simultaneously to the ballast water, the formic acid is used in a quantity of 10 to 1000% by weight, based on the sum of peracetic acid and acetic acid. In the working examples the formic acid is added to equilibrium peracetic acid in an amount of about 800% by weight, based on the sum of peracetic acid and acetic acid.
Peroxy acid solutions are most commonly prepared via an equilibrium reaction of hydrogen peroxide with the appropriate carboxylic acid. In the case of peracetic acid and carboxylic acids having higher molecular weight, an acid catalysis is required in order to reach the equilibrium in an appropriate period of time. Mineral acids, for example sulphuric acid, hydrochloric acid etc. are commonly applied as acid catalysts in such reactions. In the case of formic acid, an additional acid catalyst is not necessary according to the literature (Jones, C. W., “Applications of hydrogen peroxide and derivatives”, Royal Society of Chemistry; Clean Technology Monographs, 1999, pp. 61-77).